False positives when checking already existing source

[scb261]

Sometimes issue body contains old URL in its body, and the issue is closed as a duplicate. Most often it's because source name contains old URL in its name, and sometimes user mentions old URL in other details.

Should take substring of the body after Source new URL and before Other details.

Examples of issues with this problem:

• https://github.com/tachiyomiorg/tachiyomi-extensions/issues/16893
• https://github.com/tachiyomiorg/tachiyomi-extensions/issues/16802
• https://github.com/tachiyomiorg/tachiyomi-extensions/issues/16359
• https://github.com/tachiyomiorg/tachiyomi-extensions/issues/16475

Note: consider that the check also applies to new source requests that don't have Source new URL section.